Process and materials for marking plastic surfaces

ABSTRACT

Single coatings onto non-absorbent substrates on which aqueous inkjet inks are jetted with subsequent application of heat or other forms of energy to further cross-link the coating onto the substrate and to fuse the ink-jet ink image to give good quality water resistant colored reproductions. Optionally a protective layer is applied on the ink-jet printed image.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority from and is related to U.S.Provisional Patent Application Ser. No. 60/444,184, filed 3 Feb. 2003,this U.S. Provisional Patent Application incorporated by reference inits entirety herein.

FIELD OF INVENTION

The present invention relates to methods and compositions for providingsuitable substrate coatings for printing on plastic surfaces,specifically of containers, with aqueous ink jet inks.

BACKGROUND TO THE INVENTION

Packaging of all types of materials may require properties appertainingto functions involved in containing items and information about the useof such items. The container must have properties suited to the demandsof the product. For instance, if the product is a liquid, then thecontainer should be sealed to avoid spillage. If the liquid product isto be drunk, then the inside of the container must not contaminate theliquid nor cause its deterioration. Information of the product may beintegral to the container or it may be separate. It may for instancehave a function of aesthetically attracting a customer to the product orto instruct the customer how to use the product. As the product itselfis generally what is being consumed, packaging provides an expense areathat should perform its functions at a minimum cost. The printing ofsuch packaging should have an optimum content of automation for thisreason.

Modern technology has provided means of producing decorative andinformative patterns of information on computers and it is desirablethat this information be downloaded directly onto the packaging. Onemethod of digital printing, which may be suitable for such applications,is inkjet. Inkjet is a non-impact printing process whereby ink issquirted through very fine nozzles and the resultant ink droplets forman image directly on a substrate. There are two main types of inkjetprocess. In one process, usually termed continuous inkjet printing(CIJ), a stream of ink drops are electrically charged and then deflectedby an electric field either directly or indirectly onto the substrate.In the second process, usually called Drop on Demand (DOD) inkjetprinting, the ink supply is regulated by an actuator such as apiezoelectric actuator. The pressure produced during the actuationforces a droplet through a nozzle onto the substrate. Inks for DODinkjet printing do not need to be conductive.

WO 97/27053 by Jennel et al describes the use of inkjet to digitallywrite on packaging material. The printing can be done directly onto apre-formed bottle such as one made from PET (polyester), or onto acarton blank or a web of packaging material. The invention is claimed toprovide an advanced level of automation with minimum operatorintervention. In order to achieve good adhesion to materials such asPET, ultra-violet (UV) sensitive inks are used and after jetting theyare cured by UV radiation. The inkjet head is DOD and described as onesupplied by the company Spectra. This is the most widely accepted way ofusing UV curing inkjet inks, as the alternative method, CIJ, generallyuses water based inks and the inks must contain electrically conductivematerial. UV inks are generally based on organic acrylate mixtures thatdo not contain electrically conductive ingredients and are thereforeless easily adapted for use in CIJ.

UV inkjet inks are more expensive than water-based inks and will remainso because by definition water-based inks contain a large quantity ofwater, which is relatively inexpensive. Because with UV inkjet inks allof the jetted material remains on the substrate surface (where thesubstrate is impermeable) inks are deposited in the form of tinyhemispherical structures. Process color work, where three or fourseparate inks are applied over the same area, can thus have aBraille-like feeling and such an effect limits print quality.

The use of water-based inks in packaging applications would beadvantageous for several reasons. As has been mentioned above, they havecost advantages; they can be used in both DOD and CIJ inkjet systems andthey do not pile-up because the major part of the inkjet drop is water,which disappears either by absorption, if the substrate is pervious, orby evaporation or both. However, there are a number of problems withusing water-based inkjet inks in packaging. They have wetting problemswith relatively low energy plastic surfaces (for instance that of PET)as well as slow drying, which for non-absorbent plastic surfaces has tooccur only by evaporation. Also they have low wet smear resistance—i.e.after they have dried, they can be easily smeared with a wet finger. WO99/21724 by Wang et als. addresses the problem of ink smearing. Thepatent application describes the use of two layers—an innernon-cross-linked hydrophilic coating and an outer cross-linkedhydrophilic coating. In one embodiment, an inkjet image is appliedbefore curing to avoid wet smear. Similarly, US 2001/0036552 by Otani etal. describes coating a substrate with two layers for water-basedpigment inks to give better colors and image fastness.

It would be advantageous if a means could be found to produce a singlecoat for the substrate, which would be used to produce good qualitywater-fast images using aqueous inkjet inks.

SUMMARY OF INVENTION

This invention describes single coatings onto non-absorbent substrateson which aqueous inkjet inks are jetted with subsequent application ofheat or other forms of energy to further cross-link the coating onto thesubstrate and to fuse the ink-jet ink image to give good quality waterresistant colored reproductions.

In one aspect of the present invention, substrate coatings are describedthat are primarily designed for use in automated packaging. Suchcoatings are single layers and are initially hydrophilic, preferablyturning hydrophobic after aqueous inkjet imaging and curing. Thechanging nature of the coatings on curing permits both the initialabsorption of the aqueous inkjet ink into the coating, followed by thefixing of the ink into the cross-linked matrix, resulting in very highrub resistance and where the coating becomes hydrophobic water rejectionof the entire surface.

In an alternative less automated embodiment, the substrate may bemanufactured with the single hydrophilic coating. This substrate is thensupplied to the customer who may then image it with aqueous inkjet inkfollowed by heating to further cross-link the coating and fix the inkjetink. In such an application, the substrate together with its coatingmust be able to be handled and the coating itself must have a shelf lifeof at least several months to allow time for distribution and use.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the application as designed for automationdoes not require the initial hydrophilic coating to be handled andtherefore it may be of a fragile nature. As this is not the case withinkjet substrates as designed for non-automated systems, there islatitude in formulating such substrates, which is wider than usual.Generally, substrate coatings are subject to handling—whether duringmanufacturing, packaging of the coatings or in the actual imagingprocess. For instance, they may require resistance to absorption ofmoisture from fingers and this would therefore restrict the content ofhydrophilic constituents in the coating. In the present application,such coatings, whilst having characteristics of a solid film, may havepoor adhesion to the substrate until after imaging and curing, whenexcellent adhesion can be achieved. Also, the coatings may only have a“shelf life” of a matter of minutes, as they may be immediately used andprocessed as described herein. The coatings may be applied to a widerange of substrates, but are particularly suitable for plastics such aspolyester (PET) and polyvinyl chloride (PVC). The coating may bepigmented or transparent, depending on the application. A substrate witha white pigment, either Incorporated therein or incorporated in thesubstrate coating, has wide application as it provides an essentialbackground for transparent process inks. In the case of a coating, itcan be applied to a designated area of the substrate and the inkjetinking can be used in just this area. This Is particularly useful forbottles of drink where the color of the drink can be seen through thetransparent parts of the bottle and the bottle can still have anattractive aqueous inkjet image affixed to a white area provided by thesubstrate coating.

It is preferable but not essential to deposit the substrate from aqueoussolution.

It is also preferable, depending on the application, that the substratematerials are chosen from only those approved for food items, makingtheir use in the food and drink industry applicable.

The method of application as applied to an automated production line isas follows:

-   -   apply the solution of substrate coating to the plastic surface;    -   air-dry to evaporate the water or solvent;    -   apply the aqueous inkjet inks in the form of the required image;        and    -   treat with heat or another form of energy to cross-link and fix        the inkjet image into the substrate.        This method will be referred to below as the “automated” method.

In the less automated form, the substrate coating and initial drying maybe done as a manufacturing process for supplying the coated substrate toa customer. The customer will then image the coated substrate with theaqueous inkjet inks and cross-link them to produce the finished item.This method will be referred to below as the less automated method.

Both above described methods can be applied not only to bottles andother packaging items, but also to other inkjet applications such asprinting on credit cards, smart cards etc. If necessary, a laminate orprotective layer can be applied for extra protection against surfacedamages.

It is part of the invention that the color quality of the inkjet imageis retained or achieved after the applied energy finishing stage hasbeen completed to finalize the cross-linking process. Aqueous inkjetinks may be based on dye colorants or pigments and may containtechnologies to enhance drying and wet strength. Thus, although, withsome aqueous ink formulations it may be possible to insolubilize them onuncoated plastic substrate, because of surface energy considerations,image quality may be lost completely as the inks often reticulate onplastic surfaces.

Suitable substrate coating formulations may be water-based mixtures ofpolyvinyl alcohol and polyacrylic acid together with a water basedemulsion containing a hydrophobic polymer in the internal phase andstabilized at a pH of 7 or less. In addition, a water-solublecross-linker such as an aminoplast is used together with a catalyst. Inthe embodiments where the coated substrate is manufactured prior tosubsequent final inkjet imaging and thus must have shelf life, thecatalyst should be one only activated when energy is applied. An exampleof a suitable catalyst is an amine hindered para toluene sulfonic acid.

In all of the above applications, the mixture can also contain titaniumdioxide or a mixture of white or opaquing pigments dispersed therein.Mixtures of use in this invention, when deposited on a plastic substratecan be dried with warm air and give solid films. Deposition may be byspraying or by any other suitable means of coating.

Suitable formulations may be also solvent-based mixtures of cellulosederivatives such as hydroxypropyl cellulose, which may be deposited fromsolvent mixtures such as alcohol/ethyl acetate mixtures. The formulationshould contain solvent soluble aminoplast cross-linkers and appropriatecatalysts. They may be deposited and treated in a similar fashion to thewater based coatings and can be used for the automated or less automatedapplications.

A third type of formulations may be based on pre-polymeric mixtures thatin the presence of photo-initiators can be polymerized by ultra-violetlight. In order to achieve the initial hydrophilic properties, suitablehydrophilic polymers and extenders may be added. It is also important toarrive at a solid film before the deposition of the inkjet inks. Thistype of formulation can also be used for both the automated and the lessautomated applications.

The following examples illustrate the processes as described. Allformulations are given by weight.

EXAMPLE I

Polyvinyly alcohol solution (12% in water) 22.4 Deionized water 115Polyacrylic acid (35% in water) 32 Super Wetting Agent (Q2-5211Manufactured by 4 Dow Corporation, Midland, MI, USA) Walpol 40-136Vinyl-acrylic latex polymer 37 (Reichold Inc., Research Triangle Park,NC, USA) Cymel UFR-60 Methoxymethyl methylol urea by Cytec 13.5Industries, Five Garret Mountain Plaza, West Patterson, NJ, USA) Cycat4045 (amine inhibited toluene sulfonic acid) 8 Cytec Industries, FiveGarret Mountain Plaza, West Patterson, NJ, USA)

The above-enumerated mixture was made up and high-speed stirred. A175-micron polyester loaded with barium sulfate to give a white opaqueappearance was used as the substrate. This was coated with the aboveformulation solution using a Mayer rod and the coating was air driedovernight at room temperature. The film formed on the polyester had acoating weight of approximately 2.6 grams per square meter. This waspassed through an Epson 7600 and a colored image using aqueouspigment-based “Ultrachrome” inks was deposited on the coating. Theresulting print was then heated for 4 minutes at 140° C. and gave a highquality water-fast reproduction.

EXAMPLE II

Polyvinyly alcohol solution (12% in water) 22 Deionized water 111Polyacrylic acid (35% in water) 35 Super Wetting Agent (Q2-5211Manufactured 4 by Dow Corporation, Midland, MI, USA) Walpol 40-136Vinyl-acrylic latex polymer 37 (Reichold Inc., Research Triangle Park,NC, USA) Cymel UFR-60 Methoxymethyl methylol urea by 14 CytecIndustries, Five Garret Mountain Plaza, West patterson, NJ, USA) Cycat4045 (amine inhibited toluene sulfonic acid) 8.6 Cytec Industries Kronos2065 (Kronos Inc. Huston, Texas, USA) 53.8 Ethanol 80

The above-enumerated mixture was made up and ball-milled overnight. A175-micron transparent polyester was used as the substrate. This wascoated with the above formulation solution using a Mayer rod and thecoating was air dried overnight at room temperature. The film formed onthe polyester had a coating weight of approximately 8.7 grams per squaremeter. This was passed through an Epson 7600 and a colored image usingaqueous pigment-based “Ultrachrome” inks was deposited on the coating.The resulting print was then heated for 4 minutes at 140° C. and gave ahigh quality water-fast reproduction.

EXAMPLE III

Glasscol C44 (styrene/acrylic copolymer emulsion 8.7 sold by CibaSpeciality Chemicals, Macclesfield, UK)) Water 15.7 BYK 346 (surfactantsold by BYK-Chemie GmbH, 0.3 Postfach, Germany. Polyvinyl alcoholsolution (12% in water) 2.0 Kronos 2065 (titanium dioxide sold by KronosInc., 6.6 Huston, Texas, USA Cabosil M5 (Untreated fumed silica sold byCabot Corporation, 0.94 Tuscola, IL, US) The above enumerated mixturewas made up with stirring after each addition and ball-milled overnight.5 grams of the above mixture was then mixed with the followingingredients: Water 3.2 UFR-60 (aminoplast by Cytec Industries, WestPatterson, 0.86 NJ, USA) Cycat 4045 (catalyst by Cytec Industries, WestPatterson, 0.19 NJ, USA.)

The final mixture was coated on a 175-micron clear polyester film. Thecoating mixture was applied using a wire wound rod and was dried at 110°C. for 4 minutes to a dry weight of approximately 12 grams per squaremeter. The sheet was then imaged in an Iris Realist 2-Print prooferwhich uses dye based aqueous inks with a full process color image. Theprint was warmed for one minute at 110° C. and then coated with thefollowing solution: Daotan VTW 6462 w/36WA(self-cross-linking aliphatic14.5 urethane acrylate hybrid sold by Solutia, St.Louis, MO, USA.) BYK346 0.14

The coating mixture was applied with a wire rod and cured in the oven at140° C. for 4 minutes. The dry weight of the over-coat was approximately5 grams per square meter. Although the coating was applied to the drieddye-based aqueous inks by pressing the wire rod in contact with thesurface, surprisingly no bleeding nor smudging was observed eitherduring coating or during drying/curing. The resulting print was highgloss and high solvent resistance. For instance, the finished print wassoaked for 24 hours in 70% by weight isopropanol (25% water). Afterdrying, the print showed no damage and could be rubbed without effectingthe material and image. Other emulsion top-coats were used. It was foundthat self cross-linking urethane acrylate hybrids gave the best solventresistance.

EXAMPLE IV

The mixture of Example III was made up with one difference. Cycat 4045was replaced by Cycat 4040 (Cytec Industries)—a toluene sulfonic acidsolution that acts as a catalyst for cross-linking amoniplasts. Themixture was sprayed onto a PET bottle to give an even coating ofapproximately 12 grams per square meter. The bottle was air-dried atambient conditions and provided a white opaque ink jet receptivesurface. It was then imaged using a Iris ink jet heads mounted on alathe with the bottle fixed to rotate close to the ink jet heads. A goodquality image was obtained and this was warmed with an air gun to drythe image. It was then sprayed with the overcoat of Example III andagain dried with the air gun. No image bleeding was observed. The bottlewas immediately scuff resistant but was left for a week during which theundercoat continued to harden by means of the aminoplast cross-linkingwith the polyvinyl alcohol in the presence of the toluene sulfonic acidcatalyst.

EXAMPLE V

The following formulation was made up by weighing out each ingredientinto a bottle in the order shown and stirring the mixture after eachaddition; NeoRad R-440 (Aliphatic urethane water dispersion by 110Avecia, Neoresins, Wilmington, MA, USA.) Escacure KTO46 (photoinitiatormixture by Lamberti s.p.a. 3.2 Produti Chimici, Gallarate VA, Italy)Starch 4.8 PVP-K15 (polyvinyl pyrollidone by ISP Europe, Surrey,England) 4.8 BYK 346 2.2 Ti-Pure 746 (Titanium dioxide dispersion by 1.6Titanium Technologies, Wilmington, DE, USA) Ludox TM-40 (colloidalsilica - 40% in water sold by 80.6 Sigma-Aldrich Chemical Company,Milwaukee, USA) Glasscol C44 11.1

The completed mixture was stirred and then coated onto a 175-micronwhite polyester and dried at 110° C. for a minute, to a dry thickness ofapproximately 12 grams per square meter.

The coated polyester was imaged with a multicolored image through anEpson C82 Stylus ink jet printer. The image was warmed (1 minute at 110°C.) to drive any water either into the coating or away from the coatingby evaporation as well as to drive humectants in the inks into thecoating. The imaged material was then further coated with a 100% solidsUV-sensitive lacquer and the entire composite subject to UV light tocure both the undercoat and the overcoat, as well as to fix the imagefirmly within the system. Alternatively, the UV lacquer was omitted andthe imaged material was UV cured to cross-link the entire coating,trapping within the coating the dried ink jet inks.

1-44. (canceled)
 45. A method of producing ink-jet printed images on plastic surfaces, comprising the steps of: a. providing a plastic object; b. coating at least part of the surface of the plastic object with a mixture containing a chemically cross-linkable system; c. evaporating off the volatile part of the mixture, thus depositing a solid hydrophilic layer on the surface of the plastic object without causing the coating to cross-link; d. providing an ink-jet ink comprising a non-reactive colorant and aqueous carrier; e. jetting the ink by means of an ink-jet system onto the surface of the coating on the plastic object so that it is absorbed into the coating; f. providing energy in sufficient quantity to the printed surface to cross-link the entire surface coating including the absorbed ink-jet inks to form a non-smearable print.
 46. The method according to claim 45, wherein the coating mixture is water-based.
 47. The method according to claim 45, wherein the coating mixture contains a mixture of hydrophilic polymers and hydrophobic polymers.
 48. The method according to claim 46, wherein the coating mixture comprises polyacrylic acid polymer, and a hydrophobic resin emulsion with a pH of less than 7, together with a cross-linking substance.
 49. The method according to claim 46, wherein the coating mixture includes one of polyvinyl alcohol and hydroxy-alkyl cellulose.
 50. The method according to claim 46, wherein the cross-linkable system comprises a water-soluble amino-plast.
 51. The method according to claim 50, wherein the coating mixture contains cross-linking catalyst.
 52. The method according to claim 50, wherein the cross-linking catalyst is latent and is released during the post-imaging heating stage.
 53. The method according to claim 45, wherein the plastic is one of polyester, polyvinyl chloride, polyethylene and polypropylene.
 54. The method according to claim 45, wherein the hydrophilic layer becomes hydrophobic after ink-jet imaging and cross-linking.
 55. The method according to claim 45, wherein the ink-jet ink colorant is one of a dye and a pigment.
 56. The method according to claim 48, wherein the hydrophobic resin emulsion contains an acrylic polymer or co-polymer.
 57. The method according to claim 45, wherein the coating includes a pigment.
 58. The method according to claim 57, wherein the pigment is white.
 59. The method according to claim 58, wherein the white pigment is titanium dioxide.
 60. The method according to claim 45, wherein the coating is transparent.
 61. A plastic object coated and imaged according to the method of claim
 45. 62. A plastic object according to claim 61, wherein said object is one of a card and a bottle.
 63. The method according to claim 45, wherein the volatile portion of the coating mixture is non-aqueous.
 64. The method according to claim 63, wherein the non-aqueous coating mixture comprises: organic solvent-based mixtures; polyacrylic acid; a cross-linkable resin; a catalyst; and a hydrophobic polymer.
 65. The method according to claim 64, wherein said non-aqueous mixture additionally comprises a solvent soluble hydroxyl alkyl cellulose.
 66. The method of claim 64, wherein the cross-linkable resin is one of an aminoplast and a phenol-formaldehyde resin.
 67. The method of claim 65, wherein the cross-linkable resin is one of an aminoplast and a phenol-formaldehyde resin.
 68. The method of claim 64 wherein the hydrophobic polymer is an acrylic thermoplastic polymer or co-polymer.
 69. The method of claim 65, wherein the hydrophobic polymer is an acrylic thermoplastic polymer or co-polymer.
 70. The method according to claim 45, additionally comprising the steps of: coating the imaged layer with an over-coating mixture; and heating said over-coated surface to remove carrier liquid and cause cross-linking, to form a protective over-layer film.
 71. The method of claim 70, wherein the additional over-coating mixture is a water-based emulsion and the heating step removes the water.
 72. The method of claim 70, wherein the additional over-coating layer is a water-based cross-linkable mixture and the heating step removes water and cross-links said over-coating.
 73. The method of claim 72, wherein said mixture is an emulsion.
 74. The method of claim 45, wherein said coating mixture additionally comprises a mixture of white or opaqueing pigments dispersed therein and fillers.
 75. The method of claim 70, wherein said over-coating mixture comprises: water-soluble amino-plasts and acid catalysts.
 76. The method of claim 70, wherein said over-coating mixture comprises: water-based emulsions.
 77. The method of claim 70, wherein said over-coating mixture comprises a water-based emulsion and a cross-linking agent.
 78. The method of claim 45, wherein the entire process comprises a single automated composite action, to the extent that at no stage involves human handling.
 79. The method of claim 70, wherein the entire process comprises a single automated composite action, to the extent that at no stage involves human handling.
 80. The method according to claim 45, wherein the coating is prepared industrially and supplied in sheet form for imaging and over-coating by the user.
 81. The method according to claim 70, wherein the coating is prepared industrially and supplied in sheet form for imaging and over-coating by the user.
 82. A plastic card or bottle coated and imaged according to the method of claim
 70. 83. A method of producing ink-jet printed images with high resistance to physical and chemical damage on plastic surfaces, comprising the steps of: a. providing a plastic object; b. coating at least part of the surface of the plastic object with a solution containing a mixture of a hydrophilic polymer or polymers together with a hydrophobic polymer; c. evaporating off the volatile part of the coated solution, thus depositing a solid film on the surface of the plastic object; d. providing an ink-jet ink comprising a colorant and aqueous carrier; e. jetting the ink by means of an ink-jet system onto the surface of the coating on the plastic object; f. warming the printed surface to drive part of the water in the ink into the surface coating and to evaporate the other part of the water, as well as causing cross-linking of the deposited layer; g. over-coating the dried surface with a water-based coating; and h. heating the over-coating to seal-in the previously deposited ink and coating.
 84. A translucent or transparent bottle, imaged according to the method of claim 45, wherein said ink-jet image comprises a color image, additionally comprising a white under-layer, said color image viewable without influence of any contents of said bottle. 